You can get an price estimate on the laposte website, but it's only in french so... :1 cd with jewel case = 90g (around)For 7 cds with jewel cases, 630g, it will cost around 8.75€ , and 11.55€ registered (in a simple letter, but I'm not sure that you can put 7 jewel cases in a letter...).Without the jewel cases, I don't have a scale, but let's say around 25g per cd, 175g for 7 cds, it will cost 4.10€, and 6.90€ registered.

Those prices can be higher if the cds are sent in colissimo (over 3kg you can't send it in letter, only in colissimo. It provides more insurrances in case of lost). More than 15€.

I think that you will see the price he has paid when you will receive the package. So you will be able to open a dispute on paypal/ebay if you want, and prove that the shipping price you paid was not the real one.